Definition of "telescope"

Etymonline says that the word telescope comes from the Greek words tele, meaning far, and skopos, meaning watcher.

And in fact, I can't think of anything called a telescope that isn't used for observing something far away. But to make it a more precise definition, you should add that it observes it by using some kind of radiation, since otherwise a Mars lander would qualify as a telescope, which it clearly shouldn't.

The Collins Dictionary has the definition

2. any instrument, such as a radio telescope, for collecting, focusing, and detecting electromagnetic radiation from space.

So they have updated their definition.

However, we now have telescopes that use neutrinos and gravitational waves, and these don't focus the radiation in any meaningful sense, so removing the words "electromagnetic" and "focusing" from the definition would probably be a good idea.

In astronomy a telescope isn't used so much to make things appear closer as to concentrate a weaker signal to detect fainter objects.

The point of a telescope is to have a larger aperture than your eye and so to collect more light and allow you to see fainter, more distant objects - so Peter Shor's answer is the best.

The Collins definition is a bit off - it doesn't have to be electromagnetic (we have neutrino telescopes) and it doesn't have to focus (interferometers e.g. radio telescopes don't).

It is possible that telescope is assumed to refer to a device that operates at visible frequencies, since that is what telescopes originally did. Telescopes that detect frequencies beyond visible spectrum are usually referred to by the frequencies that they detect, e.g. microwave telescopes, radio telescopes, infra-red telescopes, etc. However, they all detect electromagnetic radiation (i.e. photons), albeit at different frequencies (or wavelengths).

Note that dictionary definitions may be somewhat vague:

radio telescope noun [ C ]

a device for receiving, for scientific study, the electromagnetic waves sent out by objects in space such as stars

Cambridge Dictionary

This definition is a little misleading, since radio telescope normally refers to a device that operates at frequencies below the infra-red spectrum.

Like microscope, the term telescope is a reasonable catch-all, although its meaning may be misunderstood.

Note that there are such things as neutrino telescopes. Neutrinos are not a form of electromagnetic radiation, although they are fundamental particles.

I think you need to refer to a specialist dictionary. I have a copy of The Penguin Dictionary of Physics copyright 1975 (abridged 1977).

Under telescope, definition 1 starts An optical device for producing a magnified or intensified image of distant objects and runs to 4 paragraphs, mentioning refracting and reflecting types.

Definition 2 is of interest here, and I quote it in its entirety:

An electronic device for detecting and amplifying electromagnetic radiation outside the visible region of the spectrum, including the *radio telescope and *electron telescope

(the asterisks indicate terms defined elsewhere in this dictionary). Note the lack of references to focussing in this definition, thus it would include even phased array telescopes like some of the square kilometre array.

For those puzzled by the electron telescope (as I was) it used a photocathode, high-voltage accelerating grids, and a phosphor screen to image UV and IR.